This invention relates to aspirating and dispensing small volumes of liquids. In particular, it relates to automatic aspirating and dispensing of small volumes of liquids onto porous brittle substrates.
The advances in biochemical technology have led to development of miniature reaction sites generally located on brittle, thin wafers, having hundreds of such reaction sites, each capable of holding small chemical and/or biological samples. The wafers are porous, with narrow pores extending into the wafer and generally normal to the plane of the surface. In order to deposit the sample onto a selected reaction site, the tip of the dispenser must be brought in close proximity with the wafer. Placing the tip of the dispenser close to the surface of the wafer introduces the risk of the tip touching the surface of the reaction medium. Since the reaction media are generally brittle, any contact could break the wafer and render useless all samples deposited on that wafer. If the contact does not break the wafer, the tip of the dispenser can abrade a coating on the vessel or destroy its confirmation. Contact with the wafer can also cause the liquid to spread on the surface. Therefore, there is a need for a system and method for dispensing small quantities of liquids containing biological and/or chemical substances in a precise location on brittle wafers without having to bring the dispenser tip into close proximity with the reaction site.
Another disadvantage of conventional methods for dispensing liquids onto wafers is that the drop at the end of the dispenser tip is placed in contact with the surface of the wafer. In order to deposit the sample in a precise location on the wafer, a drop of liquid is formed at the tip of the dispenser over the surface of the reaction site. The contact between the drop and the reaction site causes the drop to separate from the dispensing tip. The transfer of a drop of sample liquid in this manner is difficult to control because surface tension effects at the dispenser tip and the wafer surface affect the amount of liquid dispensed. As a result, there is a need for a method and system of precisely depositing small amounts of liquid at specific locations of a reaction medium. It is also necessary to have a means of dispensing liquids where the size of the drop is accurately controlled and not a function of the properties of the liquid and substrate.
One object of the present invention is to provide a system and method for accurately aspirating and dispensing submicroliter volumes of liquid onto a reaction site of a wafer without bringing the drop in contact with the surface of the wafer. Another object of the present invention is to provide a system and method for accurately verifying the volume of liquid dispensed onto the substrate.
Still another object of the present invention is to provide a system and method for dispensing subnanoliter droplets of liquid by ejecting them onto reaction sites with pore sizes 10 to 10,000 times smaller than the diameter of the drop.
Yet another object of the present invention is to provide a system and method for accurately depositing micron size droplets of liquid onto a porous substrate having submicron size pores.
Still another object of the present invention is to provide a system and method for ejecting droplets of liquid with diameters of less than 100 microns onto porous substrates with pore sizes 10 to 10,000 times smaller than the diameter of the drops. The size of the spot created by the drop on the substrate is only slightly larger than the diameter of the drop.
A further object of the present invention is to provide a system and method for aspirating and dispensing microvolumes of liquid onto porous reaction sites and accurately measuring the amount of liquid dispensed, regardless of the properties (e.g., viscosity or hydrophilicity) of the transfer liquid.
Another object of the present invention is to provide a system for aspirating and ejecting microvolumes of liquid containing chemically or biologically active s substances onto a porous reaction site of a wafer.
A still further object of the present invention is to provide for a real time monitoring of the dispensing of single 100 micron or smaller drops of liquid onto porous reaction sites of wafers.
Still another object of the present invention is to eject a plurality of drops of liquid onto a porous reaction site of a thin wafer.
Yet another object of the present invention is to eject onto a porous reaction site at least one small drop of liquid and measure, in real time, the volume of the dispensed liquid.
Other objects and advantages of the present invention will be apparent to those skilled in the art upon studying this application.
In accordance with one aspect of the present invention, 1 to 100 micron range drops of liquid are accurately deposited onto a porous reaction site having pores about 10 to about 10,000 times smaller than the size of the drop. The drops are dispensed by ejection from a tube using a piezoelectric element where the distance of the tip of the tube to the surface of the wafer is greater than the diameter of the drops. Accordingly, the drops do not touch the surface of the wafer prior to being ejected. Therefore, the properties of the liquid and the surface of the wafer do not affect the size of the drop that is ejected. The ejected drop forms a spot which is nearly the same diameter as that of the ejected droplet because it penetrates the narrow pores of the wafer.
In one aspect of the invention, a system and method for aspirating and ejecting subnanoliter drops of liquid onto a porous reaction site and detecting a pressure change resulting from the droplet ejection is presented. A known volume of a compressible fluid, e.g., a gas such as air, facilitates measuring small changes in system pressure which correlate to the volume of the transfer liquid which has been dispensed.
In accordance with another aspect of the present invention, a system and method for aspirating and ejecting subnanoliter drops of liquid onto a porous reaction site, detecting a pressure change resulting from ejection of a drop of a transfer liquid, and generating an electrical signal which indicates that single drops of liquid are dispensed at millisecond intervals is presented. By eliminating all compressible fluids (gases) from the liquid in the system, the ejection of picoliter size drops can be detected by the present invention. The dispensed drops are generally in the range of from about 5 to about 500 picoliters, often about 100 to about 500 picoliters. The pores of the wafer are in the submicron range.
In accordance with yet another aspect of the present invention, subnanoliter droplets of liquid are ejected onto porous sites of a thin wafer and the volume of the drops is measured in real time. Electrical signals indicating transient pressure changes in the transfer liquid upon dispensing liquid drops (in the range of from about 5 to about 500 picoliters, preferably about 100 to about 500 picoliters) can be detected when the liquid in the enclosed volume of the dispenser is connected to a liquid reservoir. As long as substantially all compressible fluids (gases) are kept out of the dispensing conduit (which communicates through a restricted passage to the liquid reservoir), the pressure sensor of the system of the present invention can detect dispensing a single drop of liquid in the range of from about 5 to about 500 picoliters, preferably about 100 to about 500 picoliters. The pressure change resulting from ejection of such a drop occurs in a time period long enough for the pressure change to be detectable, but short enough to complete the cycle before the next drop is ejected.
Other aspects of the present invention will become apparent to those skilled in the art upon studying this disclosure.